Modern e-beam systems used to write reticles and to direct-write wafers are required to maintain the focus and positional accuracy of the beam at the target substrate to very high accuracy (&lt;10 nm for a 4 GB DRAM reticle) over the entire substrate (commonly 6 in.sup.2) for long periods of writing time (about 4 hours for the same reticle). As the substrate is mechanically moved under the beam from field to field, the height of the target may vary by a significant amount (several microns) which requires that the beam be refocused. This focus change must be accomplished at high speed without beam motion, which would spoil the location accuracy. The beam is typically centered in the main focus lenses, but their inductance is too high to be used for fast adjustment. Usually, a separate small, low inductance coil (referred to as a dynamic focus coil) is used to make these small focus corrections, but centering the dynamic focus coil to the beam axis is a very difficult task. Thus, to render the focus coil adjustment insensitive to coil position, the focus coil field that does the focus adjustment must not affect the beam position; i.e. must be centered with respect to the beam axis at all times and must have negligible hysteresis or eddy current effects.
An article in the IBM Technical Disclosure Bulletin in 1986 (Vol. 28, No. 8, January, 1986) suggests a method of placing a dynamic focus coil within the pole pieces of a magnetic lens that has not been adopted by the art. The present invention cures some flaws in that suggested solution.